@CometKeith About 2 years ago I completely rebuilt my pond. I had a very mature 100 gal. Skippy filter. My "holding tank" was a 300 gal. Rubbermaid tank. I estimated I had about 30-40 goldfish, including a bunch of small fry. I also estimated that the fish would be there several months while we worked on the pond.
I had a system worked out for transferring everything -- which had to be done twice: once moving TO the stock tank, and then moving back to the pond. That took some coordination, but got it done. With a 3000 gal pond to move the fish
back to, I wasn't too worried about the bio-filter, but I was concerned about moving 30-40 fish into the 300 gal stock tank on the initial move.
Well, during the whole ordeal (3-4 months), I didn't lose a single fish, and my water parameters all stayed well WNL in both the "holding tank" and once they were back in the pond, and there was never even a sign of "new pond" once the pond was started back. I was really surprised at the "holding tank" as I 100% expected to be doing many, many water changes due to elevated ammonia levels.......!